1. Field of the Invention
The present invention relates to a method of manufacturing a multilayered ceramic substrate suitable for mounting semiconductor devices, and a green ceramic laminate used in the manufacturing process.
2. Description of the Related Art
An electronic device such as a mobile radio communication terminal unit or the like has recently been required to have a smaller size and higher functions, and a circuit board on which various mounted components such as semiconductor devices are mounted has been required to have a smaller size and higher reliability. In order to comply with these demands, a multilayered ceramic substrate comprising a laminate of a plurality of ceramic base layers is effective. This is because the multilayered ceramic substrate permits high-density wiring, thinning of layers, an increase in frequency, etc. The multilayered ceramic substrate is generally manufactured through a firing step, but the green ceramic body composed of a ceramic raw material powder as a main component undergoes a sintering shrinkage of about 10 to 20%, and thus causes a variation of about xc2x10.5% in dimensional precision.
In consideration of the above situation, the applicant of the present invention proposed a method in Japanese Unexamined Patent Publication No. 2000-25157, in which shrinkage-inhibiting layers composed of an alumina powder, as a main component, which is substantially not sintered at the sintering temperature of the ceramic raw material powder, i.e., which is substantially not sintered at the sintering temperature of base layers, are sandwiched between a plurality of ceramic green sheets composed of the ceramic raw material powder and a glass powder as main components, and the resultant laminate is fired at the sintering temperature of the base layers.
In this method, the shrinkage-inhibiting layers inhibit firing shrinkage of the base layers in the planar direction, and the glass component of the base layers permeates into the shrinkage-inhibiting layers and densifies the shrinkage-inhibiting layers, thereby eliminating the need to remove the shrinkage-inhibiting layers after firing. Therefore, a high-precision multilayered ceramic substrate can be manufactured.
In order to improve the function and characteristics of a multilayered ceramic substrate, attempts have recently been made to laminate different types of base materials layers, such as dielectric layers and magnetic layers, and laminate insulating layers having different thicknesses. The force of constraint required for the shrinkage-inhibiting layers, i.e., the type and thickness of the shrinkage-inhibiting layers required for sufficiently inhibiting firing shrinkage in the planar direction of the substrate, depends upon the type and thickness of the base layers. For example, in order to apply sufficient force of constraint to a thick base layer, a shrinkage-inhibiting layer (for example, a thick shrinkage-inhibiting layer) having sufficient force of constraint is required.
In order to achieve a higher-density multilayered ceramic substrate, it is necessary to form, with a high density, not only a circuit pattern of built-in elements such as a capacitor, an inductor and the like, but also a circuit pattern for connecting respective built-in elements or a built-in element and a mounted component. However, in order to form various circuit patterns in the multilayered ceramic substrate, the circuit patterns of the respective base layers generally have different densities (electrode densities). Therefore, since firing shrinkage easily proceeds in a portion having a low electrode density, a large force of constraint is required for applying sufficient constraint to a base layer having a low electrode density.
In the method disclosed in Japanese Unexamined Patent Publication Application No. 2000-25157, the shrinkage-inhibiting layers are densified by permeation of the glass component from the base materials layers, thereby causing difficulties in sufficiently densifying the shrinkage-inhibiting layers when the shrinkage-inhibiting layers are thick or the base layers contain small amounts of glass component. Consequently, it is difficult to realize a multilayered ceramic substrate having excellent reliability of strength and stability.
When the base layers contain large amounts of glass component and the shrinkage-inhibiting layers are relatively thin, the above method can sufficiently achieve densification of the shrinkage-inhibiting layers, but when the base layers contain small amounts of glass component and the shrinkage-inhibiting layers are thick, it is difficult to density the shrinkage-inhibiting layers because the permeation distance of the glass component from the base layers to the shrinkage-inhibiting layers is limited.
The present invention has been achieved for solving the above problem, and an object of the present invention is to provide a high-precision, high-reliability multilayered ceramic substrate manufactured by firing a laminate of base layers and shrinkage-inhibiting layers at the sintering temperature of the base layers in which firing shrinkage in the planar direction of the substrate is suppressed and densification of the shrinkage-inhibiting layers is achieved.
In order to achieve the object, the present invention provides a method of manufacturing a multilayered ceramic substrate comprising the step of laminating a base layer composed of a ceramic raw material powder as a main component and a shrinkage-inhibiting layer composed of a sintering-resistant powder, which is substantially not sintered at the sintering temperature of the ceramic raw material powder as a main component, for suppressing firing shrinkage of the base layer, and the step of firing the resultant laminate at the sintering temperature of the ceramic raw material powder, wherein the shrinkage-inhibiting layer contains a softening fluid powder which can densify the shrinkage-inhibiting layer due to softening and flowing accompanying firing.
In the method of manufacturing a multilayered ceramic substrate of the present invention, the shrinkage-inhibiting layer contains the softening fluid powder that softens and flows during firing to densify the shrinkage-inhibiting layer due to softening and flowing accompanying firing. Therefore, particularly when the shrinkage-inhibiting layer is thick and the base layer contains a small amount of glass component, firing shrinkage in the planar direction of the substrate can be sufficiently suppressed, and at the same time, sufficient densification of the shrinkage-inhibiting layer can be achieved to obtain a multilayered ceramic substrate with high precision and high reliability.
The present invention also provides a green ceramic laminate as a manufacturing intermediate in the method of manufacturing a multilayered ceramic substrate of the present invention. The laminate comprises a base layer composed of a ceramic raw material powder as a main component, and a shrinkage-inhibiting layer composed of a sintering-resistant powder, which is substantially not sintered at the sintering temperature of the ceramic raw material powder, for suppressing firing shrinkage of the base layer, as a main component wherein the shrinkage-inhibiting layer contains a softening fluid powder which can densify the shrinkage-inhibiting layer due to softening and flowing accompanying firing.